It's to remind them that unlike the western coalition they are there at the invitation of the Syrian government
A German Luftwaffe (Air Force) general with the goods. Talking to a German paper the general reveals that Russian Su-30 and Su-35 fighter planes in Syria make regularly stalk and shadow strike craft of the western powers flying over Syria.
The general also has the explanation why the Russians make it a point to do so. They want to remind the western powers “that they, in contrast to the international anti-IS coalition, are at the invitation of the legitimate Syrian government.”
Good for the Russians. They can't blow the intruding western aircraft out of the sky (not without a World War 3), but they can remind them that according to all international norms, they really ought to.